Interconnects for solid oxide fuel cells and ferritic stainless steels adapted for use with solid oxide fuel cells

ABSTRACT

Various embodiments relate to interconnects for solid oxide fuel cells (“SOFCs”) comprising ferritic stainless steel and having at least one via that when subjected to an oxidizing atmosphere at an elevated temperature develops a scale comprising a manganese-chromate spinel on at least a portion of a surface thereof, and at least one gas flow channel that when subjected to an oxidizing atmosphere at an elevated temperature develops an aluminum-rich oxide scale on at least a portion of a surface thereof. Other embodiments relate to interconnects comprising a ferritic stainless steel and having a fuel side comprising metallic material that resists oxidation during operation of the SOFCs, and optionally include a nickel-base superalloy on the oxidant side thereof. Still other embodiments relate to ferritic stainless steels adapted for use as interconnects comprising ≦0.1 weight percent aluminum and/or silicon, and &gt;1 up to 2 weight percent manganese. Methods of making interconnects are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Application Ser. No.60/690,671, entitled “Interconnect for Solid Oxide Fuel Cells andFerritic Stainless Steels Adapted for Use as Interconnects for SolidOxide Fuel Cells,” filed Jun. 15, 2005, which is hereby specificallyincorporated by reference herein.

BACKGROUND

The disclosure generally relates to interconnects for use with solidoxide fuel cells (“SOFCs”), and which may be used in planar solid oxidefuel cells (“PSOFCs”), and ferritic stainless steels that may be used toform interconnects for SOFCs. For example, certain non-limitingembodiments disclosed herein relate to interconnects that comprise atleast one via that when subjected to an oxidizing atmosphere at anelevated temperature develops a scale comprising a manganese-chromatespinel on at least a portion of a surface thereof, and at least one gasflow channel that when subjected to an oxidizing atmosphere at anelevated temperature develops an aluminum-rich oxide scale on at least aportion of a surface thereof. Other non-limiting embodiments relate tointerconnects for SOFCs comprising ferritic stainless steel and ametallic material that resists oxidation under certain operatingconditions of the SOFCs. Methods of making interconnects for SOFCs andPSOFCs comprising the disclosed interconnects are also described.

Solid oxide fuel cells are fuel cells that are constructed entirely ofsolid-state materials. Typically SOFCs use a fast oxygen ion-conductingceramic (typically yttria-stabilized zirconia or “YSZ”) as theelectrolyte, and operate in a temperature range of about 500° C. (932°F.) to 1000° C. (1832° F.) to facilitate solid-state transport. A singleSOFC “cell” or subunit includes an anode and a cathode separated by thesolid electrolyte. Because current generation SOFCs typically operate attemperatures up to about 1000° C., the anodes and cathodes are generallyconstructed from ceramic materials to avoid environmental degradation.Both the anode and cathode layers contain a network of interconnectedpores though which gases may pass and are good electrical conductors(e.g., they exhibit essentially no ionic conductivity). In currentgeneration SOFCs, the anode is typically formed from an electricallyconductive nickel/YSZ composite (a ceramic/metal composite or “cermet”),wherein the nickel provides a continuous electrically conductive path,while the YSZ serves to reduce the coefficient of thermal expansion ofthe overall composite and to prevent the network of pores from sinteringshut. The cathode may be based on, for example, lanthanum manganate(LaMnO₃), typically doped with strontium (replacing some of thelanthanum to yield La_(1-x)Sr_(x)MnO₃) to improve its electricalconductivity. The electrolyte is typically a thin (relative to the anodeand cathode) layer of fully dense YSZ.

During operation of the SOFC cell, an oxidant (such as O₂ or air) is fedinto the fuel cell near the cathode of the cell, where it acceptselectrons from an external circuit in the following half-cell reaction:½O_(2(g))+2e ⁻=O⁻²

Oxygen ions generated in the half-cell reaction at the cathode passthrough the YSZ electrolyte by solid-state diffusion to theelectrolyte/anode interface, where they can react with a fuel, such ashydrogen, that has been introduced to the SOFC near the anode.Operationally, pure hydrogen can be used as supplied, while ahydrocarbon fuel such as methane, kerosene, or gasoline generally mustbe partially combusted, or reformed, to hydrogen and carbon monoxide.This may be accomplished within the fuel cell, aided by the highoperating temperature and by steam injection. The fuel gas mixturepenetrates the porous anode to the anode/electrolyte interface, where itreacts with the oxygen ions from the YSZ electrolyte in the followinghalf-cell reaction:H_(2(g))+O⁻²=2e ⁻+H₂O_((g))

As indicated above, this half-cell reaction releases electrons thatre-enter the external circuit. To maintain overall electrical chargebalance, the flow of electrical charge due to oxygen ion transportthrough the electrolyte from cathode to anode is balanced by the flow ofelectrical charge due to electron conduction in the external circuit.The flow of electrons in the external circuit provides an electricalpotential of approximately one volt. To generate larger voltages, fuelcells are typically not operated as single units but, instead, as“stacks” composed of a series arrangement of several individual cellswith an “interconnect” joining and conducting current between the anodeand cathode of immediately adjacent cells. A common stack design is theflat-plate or “planar” SOFC (or “PSOFC”). In a PSOFC, at least two, andpreferably more, SOFCs are stacked together in a repeating sequence,wherein each individual SOFC is separated by an interconnect positionedbetween the anode of one SOFC and the cathode of an immediately adjacentSOFC within the stack.

Depending upon the design of a PSOFC, the interconnect can serve severalfunctions, including separating and containing the reactant gases andproviding a low resistance path for current so as to electricallyconnect the cells in series. An interconnect may also be termed a“bipolar plate” or a “separator” depending upon its function(s) in thefuel cell. In general, the interconnect material must withstand theharsh, high-temperature conditions within the cells; must be suitablyelectrically conductive (including any oxides or other surface filmsthat form on the material); and must have a coefficient of thermalexpansion (CTE) that is sufficiently similar to the CTE of the ceramicelectrodes within the cells to ensure the requisite structural integrityand gas-tightness of the fuel cell stack.

Initial PSOFC designs used a doped lanthanum chromate (LaCrO₃) ceramicmaterial as the interconnect material. LaCrO₃ ceramic does not degradeat the high temperatures at which SOFCs operate and has a coefficient ofthermal expansion that substantially matches the other ceramiccomponents of the fuel cell. LaCrO₃ ceramic, however, is brittle,difficult to fabricate, and extremely expensive. To address thesedeficiencies, metallic interconnects have been proposed for use inPSOFCs. These include interconnects formed from nickel-base alloys, suchas AL 600™ alloy, and certain austenitic stainless steels, such as the300 series family, the prototype of which is Type 304 alloy. Ferriticstainless steels, such as ALFA-II™ alloy, E-BRITE® alloy and AL 453™alloy also have been proposed for use in PSOFC interconnects. Table Iprovides nominal compositions for the foregoing nickel-base andstainless steel alloys, all of which are available from Allegheny LudlumCorporation, Pittsburgh, Pa. TABLE I Composition (weight percent) AlloyNi Cr Fe Al Si Mn Other AL 453 ™ 0.3 22 bal. 0.6 0.3 0.3 0.06 alloy max.Ce + La max. E-BRITE ® 0.15 26 bal. 0.1 0.2 0.05 1 Mo alloy max.ALFA-II ™ 0.3 13 bal. 3 0.3 0.4 0.4 Ti alloy max. AL 600 ™ Ni + 15.5 8 —0.2 0.25 — alloy Co bal. Type 304 8 18 bal. — — — — alloy

At the operating temperatures typical for current generation SOFC, thepartial pressure of oxygen (or “pO₂”) near the anode of a SOFC isgenerally lower than the pO₂ required for various metals commonly usedas electrical conductors (e.g., copper and nickel) to form oxides.However, the pO₂ near the cathode of a SOFC is generally higher than thepO₂ required for oxide formation. Accordingly, there is a tendency forsurface oxide layers to form on interconnects made from these metalswhen exposed to the oxidant proximate the cathode of the SOFC.

Since metals generally form oxides that either have a high electricalresistivity at the temperatures typical of PSOFC operation or rapidlythicken with time, the area specific resistance (or “ASR”) of metalinterconnects, as well as the resistivity of the PSOFC stack into whichthey are incorporated, tends to increase with time during operation ofthe PSOFC. For example, certain alloys, upon exposure to oxygen at hightemperatures, form surface oxides that either thicken at an extremelyslow rate (for example, the Al₂O₃ scale of ALFA-II® alloy) or are highlyelectrically conductive (for example, the NiO scale of pure ordispersion-strengthened nickel). However, the underlying mechanism thatcontrols these two seemingly disparate factors (resistivity and rate ofoxide formation) is essentially the same (the electronic defectstructure of the oxide). Consequently, there are very few metal oxidesthat are both slow growing and electrically conductive.

Stainless steels have attracted interest as potential interconnectmaterials, in part, because in their conventional form they develop ascale consisting primarily of chromium oxide (Cr₂O₃). This oxide scaleis both relatively slow growing and reasonably electrically conductiveat typical, current generation SOFC operating temperatures. Ferriticstainless steels in particular have certain properties that make themattractive for PSOFC interconnect applications, including low cost, goodfabricability, and CTE compatible with ceramic. Nevertheless, theoxidation of stainless steel interconnects during operation of a PSOFCmay lead to an undesirable degradation of electrical properties of thePSOFC over time.

Another potential drawback to the utilization of stainless steels inPSOFC applications is “poisoning” of the porous electrodes, and inparticular the cathodes, used in the SOFCs by chromium-bearing vaporspecies that may evolve from the chromium-rich oxide scale on thesurface of the stainless steel during operation, particularly in thepresence of water or hydrogen. Because water vapor is often present inthe gas streams of an operational PSOFC, the formation of volatilechromium oxy-hydroxides (e.g., CrO₂(OH)₂) at lower temperatures canexacerbate the problem. Additionally, solid state diffusion of chromiumfrom the interconnect to the adjoining cathode may occur duringoperation of the PSOFC and may also contribute to cathode poisoning.While the formation of a manganese-chromate spinel layer on the surfaceof a stainless steel interconnect may reduce such chromium migration(e.g., the evolution of chromium-bearing vapor species and/or solidstate diffusion of chromium) during operation of a PSOFC, if sufficientchromium is present at the surface of the interconnect, chromiummigration leading to cathode poisoning may still occur.

Various structures have been proposed for SOFC interconnects. Forexample, U.S. Pat. No. 6,326,096 discloses an interconnect for solidoxide fuel cells having a superalloy metallic layer with a anode-facingface, a cathode-facing face, and a metal layer, preferably nickel orcopper, on the anode-facing face (see Abstract). Disclosed superalloysinclude Inconel® alloys, Haynes® alloys, Hastelloy® alloys, andaustenitic stainless steels (see col. 4, lines 60-63).

U.S. Pat. No. 4,781,996 discloses a separator plate that is laminated onthe back surface of each of the anode and the cathode of a fuel cell,and is made of an nickel-containing iron alloy containing from about25-60% nickel in order to match the linear expansion coefficient of theseparator plate with the expansion coefficient of the electrolyte plate(see col. 3, lines 18-27). Further, an oxidation resistant treatingmaterial is bonded to the cathode side of the separator and an alkalicorrosion-resistant treating material is bonded to the anode side of theseparator (see col. 4, lines 24-29).

U.S. Pat. No. 5,227,256 discloses a bimetallic separator plate for afuel cell in which stainless steel may be used on the cathode face andnickel or copper on the anode face (see col. 11, lines 34-38). Further,the nickel or copper may be about 10 percent of the thickness of theseparator plate (see col. 1, lines 38-40). Specifically disclosed are300 series stainless steel alloys (see col. 11, lines 40-42).

U.S. Pat. No. 5,733,683 discloses a metallic bipolar plate forhigh-temperature fuel cells, the plate having a body having surfacesadapted to contact electrodes of the fuel cells and passages havingwalls confining gases. The body of the plate is composed of achromium-containing alloy oxidizable at the surface to form chromiumoxide, the alloy being enriched with aluminum at least in regions of thewalls in direct contact with the gases (see Abstract). Aluminumenrichment can be carried out using a conventional aluminum diffusionprocess, wherein the plate is coated with a powder mixture of inertmaterial (such as Al₂O₃), a chloride/fluoride activator (such as NaCl)and aluminum powder, and exposed at 600° C. to 1300° C. under argon, orcoated using CVD or PVD (see col. 3, lines 43-57). Thereafter, surfacesof the plate wherein aluminum enrichment is not desired (for example theelectrical contact surfaces) are ground to remove the enriched layer ofmaterial. In order to accommodate grinding, the body of the plate isover-sized to account for material removal (see col. 3, lines 57-62).

Canadian Patent No. 2,240,270 discloses a bipolar plate consisting of achromium oxide-forming alloy with an electrically insulating, corrosionreducing layer in the region of the gas guiding surfaces and cobalt,nickel or iron enrichment layers in the region of the electrode contactsurfaces (see Abstract). As discussed above with respect to U.S. Pat.No. 5,733,683, grinding is required to remove the electricallyinsulating layer from the electrode contact surfaces; accordingly, theplate is over-sized to account for the material removal (see page 8,lines 10-15).

U.S. Patent Publication 2003/0059335 discloses a high temperaturematerial that consists of a chromium oxide forming iron alloy including12 to 28 wt % chromium, 0.01 to 0.4 wt % La, 0.2 to 1.0 wt % Mn, 0.1 to0.4 wt % Ti, less than 0.2 wt % Si, and less than 0.2 wt % Al, whereinat temperatures of 700° C.-950° C. the material forms a MnCr₂O₄ spinelphase and which can be used to form a bipolar plate for a SOFC (seeAbstract and paragraph [0032]).

There remains a need for interconnects for SOFCs that have oxidationproperties that are tailored to the environmental conditions experiencedby interconnects during operation of a PSOFC, that do not requirehigh-temperature treatments, over-sizing or the use of expensivesuperalloys to achieve desired properties, and that may provide forimproved electrical performance of the PSOFCs into which they areincorporated. Further, there is a need for ferritic stainless steelsthat are compositionally tailored for the SOFC environment and fromwhich such interconnects may be fabricated.

BRIEF SUMMARY OF VARIOUS EMBODIMENTS OF THE INVENTION

Various non-limiting embodiment disclosed herein related to a ferriticstainless steel comprising, in weight percent, from 0 to less than 0.1aluminum, from 0 to less than 0.1 silicon, from 21 to 35 chromium,greater than 1 to 2 manganese, from 0.002 to 0.1 carbon, from 0 to 0.04nitrogen, from 0 to 1 molybdenum, from 0 to 0.5 nickel, from 0 to 0.05lanthanum, from 0 to 0.1 cerium, from 0 to 0.1 zirconium, from 0 to 0.5titanium, from 0 to 0.1 tantalum, from 0 to 0.2 niobium, iron andimpurities.

Other non-limiting embodiments related to a ferritic stainless steelcomprising, in weight percent, from 0 to 0.05 aluminum, from 0 to 0.05silicon, from 21 to 24 chromium, greater than 1 to 2 manganese, from0.002 to 0.1 carbon, from 0 to 0.04 nitrogen, from 0 to 1 molybdenum,from 0 to 0.3 nickel, from 0.02 to 0.04 lanthanum, from 0 to 0.1zirconium, from 0 to 0.1 titanium, from 0 to 0.1 tantalum, from 0 to 0.1niobium, cerium, iron and impurities, wherein a sum of the weightpercent cerium and the weight percent lanthanum ranges from 0.03 to0.06.

Still other non-limiting embodiments relate to a ferritic stainlesssteel comprising, in weight percent, from 0 to 0.05 aluminum, from 0 to0.05 silicon, from 23 to 27 chromium, greater than 1 to 2 manganese,from 0.002 to 0.1 carbon, from 0 to 0.04 nitrogen, from 0 to 1molybdenum, from 0 to 0.3 nickel, from 0 to 0.05 lanthanum, from 0 to0.1 cerium, from 0 to 0.1 zirconium, from 0 to 0.5 titanium, from 0 to0.1 tantalum, from 0.05 to 0.2 niobium, iron and impurities.

Yet other non-limiting embodiments relate to a ferritic stainless steelcomprising, in weight percent, from 0 to 0.05 aluminum, from 0 to 0.05silicon, from 23 to 27 chromium, greater than 1 to 2 manganese, from0.002 to 0.1 carbon, from 0 to 0.04 nitrogen, from 0.75 to 1 molybdenum,from 0 to 0.3 nickel, from 0 to 0.05 lanthanum, from 0 to 0.1 cerium,from 0 to 0.05 zirconium, an amount of at least one of titanium,tantalum and niobium, wherein the amounts of titanium, tantalum andniobium satisfy the equation:0.4 weight percent≦[% Nb+% Ti+½(% Ta)]≦1 weight percent,iron and impurities.

Other non-limiting embodiments relate to interconnects for use withSOFCs made using the ferritic stainless steels disclosed herein andPSOFC including the same.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

Various non-limiting embodiments described herein may be betterunderstood when read in conjunction with the drawings, in which:

FIG. 1 is a schematic perspective view of an interconnect for SOFCs;

FIG. 2 is a schematic, perspective view of a PSOFC, including aninterconnect;

FIGS. 3 a, 3 b, and 3 c are schematic, cross-sectional views ofinterconnects according to various non-limiting embodiments disclosedherein;

FIG. 4 is a graph illustrating the variation in oxygen partial pressureversus operating temperature for various fuel utilization levels in aconventional PSOFC;

FIG. 5 is an Ellingham diagram on which certain operating conditions(i.e., temperature and pO2) proximate the fuel side of an interconnectduring operation of a typical planar solid oxide fuel cell are generallyindicated;

FIG. 6 is a graph of the coefficients of thermal expansion for severalmaterials;

FIG. 7 is a plot of weight change per unit surface area versus time forsamples of two different ferritic stainless steels, one of which waselectropolished and one of which was not electropolished;

FIG. 8 a is a secondary electron image of a surface of ferriticstainless steel that has been electropolished and a portion of which wassubsequently abraded; and

FIGS. 8 b-e are characteristic x-rays maps for chromium, iron, aluminum,and manganese, respectively, obtained from the same area shown in FIG. 8a.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

As used in this specification and the appended claims, the articles “a,”“an,” and “the” include plural referents unless expressly andunequivocally limited to one referent. Additionally, for the purposes ofthis specification, unless otherwise indicated, all numbers expressingquantities, such as weight percentages and processing parameters, andother properties or parameters used in the specification are to beunderstood as being modified in all instances by the term “about.”Accordingly, unless otherwise indicated, it should be understood thatthe numerical parameters set forth in the following specification andattached claims are approximations. At the very least, and not as anattempt to limit the application of the doctrine of equivalents to thescope of the claims, numerical parameters should be read in light of thenumber of reported significant digits and the application of ordinaryrounding techniques.

Further, while the numerical ranges and parameters setting forth thebroad scope of the invention are approximations as discussed above, thenumerical values set forth in the Examples section are reported asprecisely as possible. It should be understood, however, that suchnumerical values inherently contain certain errors resulting from themeasurement equipment and/or measurement technique.

As discussed above, stainless steels, and in particular ferriticstainless steels, have been considered as possible cost-effectivereplacement materials for SOFC interconnects. However, the environmentalconditions proximate both the anode and the cathode of an operationalSOFC may cause oxidation of most iron-chromium (“Fe—Cr”) containingstainless steels. Since an oxidized interconnect is generally a lessefficient electrical current conductor than a non-oxidized interconnect,the efficiency of the fuel cell stack as a whole may decrease over timeas the oxide scale on the interconnects within the fuel cell increasesin thickness. The inherent limitations of current generation metallicinterconnects has limited available SOFC and PSOFC designs to relativelyinefficient, low temperature operation (approximately 700° C. (1292°F.)) in order to prevent excessive oxidation on the interconnectsurfaces.

As used herein the term “interconnect” means a device that connects(electrically and/or mechanically) one component to another component.Further, although not required, the interconnects according to thevarious non-limiting embodiments disclosed herein may also serve toseparate and/or direct the flow of the gaseous reactants used duringoperation of a SOFC. For example, as previously discussed, immediatelyadjacent SOFCs in a PSOFC are typically connected together by aninterconnect that is interposed between the cathode of one SOFC and theanode of an immediately adjacent SOFC, provides electricalinterconnection between the SOFCs, and serves to separate and direct theflow of gaseous reactants. The interconnects according to variousnon-limiting embodiments disclosed herein may also be used to connect anSOFC of one PSOFC to the SOFC of another PSOFC. For example, theinterconnects according to various non-limiting embodiments disclosedherein can form the “end plate” of a PSOFC and can serve to electricallyand/or mechanically connect that PSOFC to an adjacent PSOFC or,alternatively, to another component in the system.

Referring now to FIG. 1, there is shown a schematic perspective view ofan interconnect (generally indicated as 10 in FIG. 1) having a typicalconfiguration for use in current generation PSOFCs. It will beappreciated by those skilled in the art that the precise design of aninterconnect will depend upon the design of the PSOFC and the individualSOFCs involved. Accordingly, the interconnect configuration shown inFIG. 1 is not intended to limit the possible configurations of theinterconnects according to the invention, but is provided forillustration purposes only. As shown in FIG. 1, interconnect 10 has afirst side 14 and a second side 16 opposite the first side. Both thefirst and second sides have at least one gas flow channel 18 and atleast one via 20. As used herein the term “via” means an electricallyconductive pathway. When positioned in a PSOFC between two adjacentSOFC, the vias on one side of the interconnect make electrical contactwith an electrode of one of the SOFCs in the stack, while the vias onthe opposite side of the interconnect make electrical contact with anoppositely charged electrode of an adjacent SOFC in the stack,permitting the flow of current from one SOFC to the next. For example,as shown in FIG. 2, interconnect 210 of the PSOFC (generally indicatedas 211) is positioned such that the vias 220 a on side 214 ofinterconnect 210 are adjacent cathode 222 of one SOFC (generallyindicates as 223), while the vias 220 b on side 216 of interconnect 210are adjacent anode 224 of an adjacent SOFC (generally indicated as 225)in the stack. As shown in FIG. 2, SOFC 223 includes cathode 222,electrolyte 230 and anode 232, and SOFC 225 includes anode 224,electrolyte 234 and cathode 236.

With continued reference to FIG. 2, during operation of a PSOFC, gasesflows through the gas flow channels 218 a, 218 b on both sides (214,216) of the interconnect 210. The gas flowing through the gas flowchannels 218 on side 216 of the interconnect 210, which is adjacentanode 224, is a fuel-rich gas, for example a hydrogen-rich gas; whilethe gas flowing through the gas flow channels 218 on side 214 adjacentcathode 222 is an oxidant-rich gas, typically air. Thus, side 214 ofinterconnect 210 is exposed to the oxygen-rich environment near cathode222 during operation of the PSOFC, and side 216 is exposed to thefuel-rich environment near anode 224 during operation of the PSOFC. Asused herein with reference to the interconnect, the term “oxidant side”refers to the side of an interconnect that is or will be adjacent thecathode of a SOFC during operation of a PSOFC into which it isincorporated, for example, side 214 in FIG. 2. Further, as used hereinwith reference to the interconnect, the term “fuel side” refers to theside of an interconnect that is or will be adjacent the anode of a SOFCduring operation of a PSOFC into which it is incorporated, for example,side 216 in FIG. 2.

Interconnects according to various non-limiting embodiments disclosedherein will now be described. One non-limiting embodiment provides aninterconnect for SOFCs, the interconnect being formed from a ferriticstainless steel and comprising at least one via that when subjected toan oxidizing atmosphere at a temperature of at least 650° C. develops ascale comprising a manganese-chromate spinel on at least a portion of asurface thereof, and at least one gas flow channel that when subjectedto an oxidizing atmosphere at a temperature of at least 650° C. developsan aluminum-rich oxide scale on at least a portion of a surface thereof,the aluminum-rich oxide scale comprising iron and chromium and having ahematite structure. As used herein the term “scale” refers to oxidationproducts at or on the surface of a material.

As previously discussed, ferritic stainless steels have been consideredfor use in SOFC interconnect applications largely due to their abilityto form an electrically conductive, chromium-rich oxide scale whenexposed to an oxidizing atmosphere (such as that seen during operationof an SOFC) and their relatively low CTEs. However, one drawback to theutilization of ferritic stainless steels in SOFC interconnectapplications is the potential for poisoning of the porous electrodes,and in particular the cathodes, used in the SOFC due to chromiummigration (e.g., the evolution of chromium-bearing vapor species and/orsolid state chromium diffusion) from the chromium-rich oxide scale thatforms during operation of the SOFC. As previously discussed, becausewater vapor is often present in the gas streams of an operational SOFC,the formation of volatile chromium-oxy hydroxides, particularly in gasflow channels of an interconnect, may exacerbate this problem.

In contrast to chromium-rich oxide scales, aluminum-rich oxide scalesare essentially unaffected by the water vapor present in the gas streamsof the operational SOFC. Further, the formation of an aluminum-richoxide scale on the surface of a ferritic stainless steel may reduce orprevent chromium migration from the surface of the ferritic stainlesssteel during operation of a SOFC. However, since aluminum-rich oxidescales have a high electrical resistivity, the formation ofaluminum-rich oxide scales on electrical contact surfaces (for example,via surfaces) is generally not desirable. While a scale comprisingmanganese-chromate spinel can form on the surface of some stainlesssteels and is a reasonably good electrical conductor, as previouslydiscussed, if sufficient chromium is present in the scale, chromiummigration leading to cathode poisoning may still occur.

Although not meaning to be bound by any particular theory, it iscontemplated that by selectively treating at least the surfaces of thegas flow channels of the interconnects according to various non-limitingembodiments disclosed herein such that an aluminum-rich oxide scale mayform on at least a portion of the treated surfaces when subjected to anoxidizing atmosphere at a temperature of at least 650° C., chromiummigration from those surfaces can be reduced without detrimentallyeffecting the ASR of the interconnect. That is, since the surfaces ofthe gas flow channels are not in direct electrical contact with adjacentelectrodes (for example as shown in FIG. 2), the formation of analuminum-rich oxide scale on the surfaces of the gas flow channelsshould not detrimentally effect the ASR of the interconnect. However,since the regions of the interconnect that are most prone to theformation of chromium-bearing vapor species are the gas flow channels,in part due to the high water vapor content of the gases flowingtherethrough, it is contemplated that the formation of an aluminum-richoxide scale on these surfaces may reduce the amount and/or rate offormation of chromium-bearing vapor species from these surfaces duringoperation of a SOFC, which may in turn reduce the occurrence of cathodepoisoning.

According to various non-limiting embodiments disclosed herein, whereinthe gas flow channel comprises at least one surface that when subjectedto an oxidizing atmosphere at a temperature of at least 650° C. developsan aluminum-rich oxide scale, the interconnect may be formed from aFe—Cr ferritic stainless steel comprising a sufficient alloy content topermit the formation of the aluminum-rich oxide scale. For example,according to various non-limiting embodiments, the interconnect may beformed from a ferritic stainless steels comprising from 0.3 to 1 weightpercent aluminum, and at least one rare earth element (such as, but notlimited to, cerium, lanthanum, and praseodymium), provided that theferritic stainless steel comprises a total of at least 0.03 weightpercent of rare earth elements. Further, to allow for the formation of ascale comprising a manganese-chromate spinel on at least a portion of asurface of a via (as discussed above), according to various non-limitingembodiments, the ferritic stainless steel may further comprise from 0.2to 4 weight percent manganese.

One specific non-limiting example of a ferritic stainless steelcomprising from 0.2 to 4 weight percent manganese, from 0.3 to 1 weightpercent aluminum and at least 0.03 weight percent of rare earth elementsthat can be used in conjunction with various non-limiting embodimentsdisclosed herein is a ferritic stainless steel comprising from 0.002 to0.1 weight percent carbon, from 21 to 35 weight percent chromium, from0.2 to 4 weight percent manganese, from 0.3 to 0.5 weight percentaluminum, from 0 to 0.05 weight percent lanthanum, from 0 to 0.1 weightpercent cerium, and iron and impurities, provided that a sum of theweight percent lanthanum and the weight percent cerium is at least 0.03.

Another non-limiting example of a ferritic stainless steel comprisingfrom 0.2 to 4 weight percent manganese, from 0.3 to 1 weight percentaluminum and at least 0.03 weight percent rare earth elements that canbe used in conjunction with various non-limiting embodiments disclosedherein is a ferritic stainless steel comprising from 0.002 to 0.1 weightpercent carbon, from 0 to 0.03 weight percent nitrogen, from 21 to 24weight percent chromium, from 0 to 0.3 weight percent nickel, from 0 to0.4 weight percent molybdenum, from 0.2 to 0.5 weight percent manganese,from 0.5 to 0.8 weight percent aluminum, from 0 to 0.5 weight percentsilicon, from 0 to 0.02 weight percent niobium, from 0 to 0.01 weightpercent titanium, from 0.008 to 0.02 weight percent lanthanum, cerium,iron and impurities, wherein the sum of the weight percent lanthanum andthe weight percent cerium ranges from 0.03 to 0.06 weight percent. Onecommercially available, non-limiting example of such a ferriticstainless steel is AL 453™ ferritic stainless steel alloy, which isavailable from Allegheny Ludlum Corporation of Pittsburgh, Pa.

Alternatively, and as discussed below in more detail, if only one sideof the interconnect (for example the oxidant side) comprises a gas flowchannel comprising at least one surface that when subjected to anoxidizing atmosphere at a temperature of at least 650° C. develops analuminum-rich oxide scale, that side or portion of the interconnect maybe formed form a ferritic stainless steel having sufficient alloycontent to permit the formation of an aluminum-rich oxide scale, whilethe other opposite side of the interconnect (for example the fuel side)may be formed from a ferritic stainless steel comprising less than 0.3weight percent aluminum. Non-limiting examples of ferritic stainlesssteels comprising less than 0.3 weight percent aluminum are set forthbelow in more detail.

As previously discussed, the interconnects according to variousnon-limiting embodiments disclosed herein can comprise at least one gasflow channel that when subjected to an oxidizing atmosphere at atemperature of at least 650° C. develops an aluminum-rich oxide scale onat least a portion of a surface thereof. Further, according to variousnon-limiting embodiment, the aluminum-rich oxide scale may comprise ironand chromium. More particularly, according to various non-limitingembodiments disclosed herein, the aluminum-rich oxide scale may comprisealuminum, iron, chromium and oxygen (wherein at least a portion of theiron and chromium cations replace a portion of the aluminum cations inthe aluminum oxide lattice structure, and may have a hematite structure.More specifically, the hematite structure may have lattice parameters a₀and c₀, wherein a₀ ranges from 4.95 Angstroms (Å) (i.e., 10⁻¹⁰ m) to5.04 Å and c₀ ranges from 13.58 Å to 13.75 Å.

In contrast to Fe—Cr—Al ferritic stainless steels, which typicallycontain from 3 weight percent to 7 weight percent aluminum, typicalFe—Cr ferritic stainless steels do not develop an aluminum-rich oxidescale on their surface when oxidized. That is, generally when typicalFe—Cr ferritic stainless steels, which contain only residual levels ofaluminum (e.g., 0.3 to 0.5 weight percent), are exposed to an oxidizingatmosphere, the Fe—Cr ferritic stainless steel tends to form achromium-rich oxide scale on its surface. Further, depending upon thecomposition of the ferritic stainless steel, a scale comprising amanganese-chromate spinel may form on at least a portion of the ferriticstainless steel. However, the inventors herein have discovered that byelectropolishing certain Fe—Cr ferritic stainless steels that containlevels of aluminum, which are otherwise insufficient to produce a nativealuminum-rich oxide scale on their surface during oxidation, analuminum-rich oxide scale can be developed on the Fe—Cr ferriticstainless steel when it is exposed to an oxidizing atmosphere at atemperature of at least 650° C.

Further, the inventors herein have discovered that the effects ofelectropolishing Fe—Cr ferritic stainless steels may be reduced oreliminated, for example, by removing material from the electropolishedsurface. Accordingly, when exposed to an oxidizing atmosphere at atemperature of at least 650° C., those regions of the ferritic stainlesssteel that have either not been electropolished or from which theeffects of electropolishing have been removed can develop a scalecomprising a chromium-rich oxide and/or a manganese-chromate spinel ontheir surfaces.

For example, although not limiting herein, it has been observed by theinventors that, after electropolishing, Fe—Cr ferritic stainless steelscontaining from 0.3 to 1 weight percent aluminum and at least 0.03weight percent of rare earth element(s), may form an aluminum-rich oxidescale on exposure to oxidizing conditions (such as those seen duringoperation of a PSOFC). Further, as discussed above, while the formationof an aluminum-rich oxide scale on certain regions of an interconnectmay be advantageous in reducing the rate of cathode poisoning due tochromium migration from those regions, because aluminum-rich oxidesscales have a high electrical resistivity, the presence of such a scaleon the via surfaces of the interconnect can lead to an increase in ASRand degrade the electrical properties of the interconnect. Therefore,according to certain non-limiting embodiments disclosed herein, theinterconnect may be treated so as to permit the formation of analuminum-rich oxide scale on certain surfaces of the interconnect, suchas but not limited to, the gas flow channel surfaces, manifold surfacesand/or sealing flange surfaces, while preventing the formation of analuminum-rich oxide scale on other surfaces, such as the via surfaces.For example, one non-limiting embodiment provides an interconnect forSOFCs, the interconnect being formed from a ferritic stainless steel andcomprising at least one via that when subjected to an oxidizingatmosphere at a temperature of at least 650° C. develops a scalecomprising a manganese-chromate spinel on at least a portion of asurface thereof, and at least one gas flow channel comprising at leastone electropolished surface. As used herein the term “manifold” refersto the portion(s) of the interconnect that connect the gas flow channelson the oxidant side and the fuel side of the interconnect to the air andfuel gas supplies, respectively. Further, as used herein the term“sealing flange” refers to the portions of the interconnect, generallyat the out perimeter of the interconnect to which a sealing compoundsuch as, but not limited to, an alkaline glass, is applied to create agas-tight seal for the PSOFC.

In another non-limiting embodiment, the surfaces of the sealing flangeof the interconnect may also be electropolished to both reduce chromiummigration from those surfaces and to provide an electrically insulatingsurface in the sealing area. Although not limiting herein, it iscontemplated that by providing an electrically insulating surface at thesealing flange of the interconnect a variety of sealing compounds,including both electrically insulating and electrically conductivesealing compounds, may be employed.

As used herein, the terms “electropolishing” and “electropolished” referto the electrochemical removal of material from at least a portion of awork piece. For example, according to one non-limiting embodiment, theentire interconnect can be electropolished, and thereafter, selectedsurfaces (such as, for example, via surfaces) of the electropolishedinterconnect can be mechanically or chemically polished, ground, etchedand/or milled to remove or abrade material from the selected surfaces,thereby reducing or eliminating the effect of electropolishing on thosesurfaces (as discussed above).

According to other non-limiting embodiments, the interconnect can besubjected to a selective electropolishing treatment such that materialis electrochemically removed from at least a portion of gas flow channelsurfaces, while essentially no material is electrochemically removedfrom the via surfaces. As used herein, the terms “selectiveelectropolishing” refers to the electrochemical removal of material fromone or more pre-selected portions or regions of a work piece. Further,as used herein with reference to various portions of an interconnect,the term “selectively electropolished” refers to those pre-selectedportions of the interconnect from which material is electrochemicallyremoved. Methods of selectively electropolishing are discussed hereinbelow in more detail.

As discussed above, during operation of an SOFC, the ferritic stainlesssteel interconnects according to various non-limiting embodimentsdisclosed herein will undergo oxidation such that the interconnect willcomprise a scale comprising a manganese-chromate spinel on at least aportion of a surface of at least one via and an aluminum-rich oxidescale on at least a portion of a surface of at least one gas flowchannel. The thickness of these various scales will depend on severalfactors, for example the steel composition, exposure time, operatingtemperature, humidity and gas composition. Although not limiting herein,according to certain embodiments, the scale comprisingmanganese-chromate spinel may have a thickness of less than 10 microns,and the aluminum-rich oxide scale may have a thickness that is less than5 microns. Further according to various non-limiting embodimentsdisclosed herein, the thickness of the aluminum-rich oxide scale mayrange from 1 micron to less than 5 microns, and may further range from 2microns to 3 microns.

Other non-limiting embodiments disclosed herein provide an interconnectfor SOFCs, the interconnect comprising a fuel side and an oxidant sideopposite the fuel side, the oxidant side of the interconnect beingformed from a ferritic stainless steel and comprising at least one viathat when subjected to an oxidizing atmosphere at a temperature of atleast 650° C. develops a scale comprising a manganese-chromate spinel onat least a portion of a surface thereof and at least one gas flowchannel that when subjected to an oxidizing atmosphere at a temperatureof at least 650° C. develops an aluminum-rich oxide scale on at least aportion of a surface thereof, the aluminum-rich oxide scale comprisingiron and chromium and having a hematite structure. For example,according to this non-limiting embodiment, at least the oxidant side ofthe interconnect can be formed from a ferritic stainless steelcomprising from 0.2 to 4.0 weight percent manganese, from 0.3 to 1.0weight percent aluminum, and at total of at least 0.03 weight percent ofrare earth element(s). Further, according to this non-limitingembodiment at least a portion of at least one surface of the gas flowchannel of the oxidant side of the interconnect can be electropolishedor selectively electropolished.

As previously discussed, typical interconnects for use in SOFCs have afuel side and an oxidant side, each of which comprise vias that permitflow of electrical charge between adjacent SOFCs in a PSOFC, as well asgas flow channels that provide a passageway for transport of a gas oneither side of the interconnect. Thus, according to this and othernon-limiting embodiments disclosed herein, the fuel side of theinterconnect can be formed from a ferritic stainless steel and cancomprise at least one gas flow channel comprising at least one surfacethat when subjected to an oxidizing atmosphere at a temperature of atleast 650° C. develops an aluminum-rich oxide scale. For example,according to this non-limiting embodiment, the fuel side of theinterconnect can be formed from a ferritic stainless steel comprisingfrom 0.2 to 4.0 weight percent manganese, from 0.3 to 1 weight percentaluminum and a total of least 0.03 weight percent of rare earthelement(s). Further, according to this non-limiting embodiment, at leasta portion of the at least one gas flow channel of the fuel side can beelectropolished or selectively electropolished. Additionally, accordingto this non-limiting embodiment, the fuel side of the interconnect cancomprise at least one via that develops a scale comprising amanganese-chromate spinel on at least a portion of a surface thereofwhen exposed to an oxidizing atmosphere at a temperature of at least650° C.

Alternatively, according to this and other non-limiting embodimentsdisclosed herein, the fuel side of the interconnect may be formed from aferritic stainless steel (which may be the same as or different from theferritic stainless steel which forms the oxidant side of theinterconnect) and may comprises a layer comprising a metallic materialthat forms an oxide having a dissociation pressure greater than apartial pressure of oxygen proximate the fuel side of the interconnectduring operation of the SOFC connected to at least a portion of theferritic stainless steel (for example, as shown in FIG. 3 a, which isdiscussed below in more detail). Still further, the fuel side of theinterconnect according to this and other non-limiting embodimentsdisclosed may be formed from a metallic material that forms an oxidehaving a dissociation pressure greater than a partial pressure of oxygenproximate the fuel side of the interconnect during operation of the SOFCand connected to the oxidant side of the interconnect (for example, asshown in FIG. 3 b, which is discussed below in more detail).

For example, one non-limiting embodiment provides an interconnect forSOFCs, the interconnect comprising an oxidant side formed from aferritic stainless steel and comprising a via (which may develop a scalecomprising at least one of chromium oxide and manganese-chromate on atleast a portion of a surface thereof when subjected to an oxidizingatmosphere at a temperature of at least 650° C.), and a gas flowchannel, the gas flow channel comprising at least one surface that whensubjected to an oxidizing atmosphere at a temperature of at least 650°C. develops an aluminum-rich oxide scale, and a fuel side opposite theoxidant side, the fuel side comprising a metallic material that forms anoxide having a dissociation pressure greater than a partial pressure ofoxygen proximate the fuel side of the interconnect during operation ofthe SOFCs. For example, according to this non-limiting embodiment, thefuel side of the interconnect can be formed from a ferritic stainlesssteel, which may be the same as or different from the ferritic stainlesssteel of the oxidant side, and may comprise a layer comprising ametallic material that forms an oxide having a dissociation pressuregreater than the partial pressure of oxygen proximate the fuel side ofthe interconnect during operation of the SOFCs that is connected to theferritic stainless steel of the fuel side. For example, a layercomprising the metallic material may be plated or clad to a portion of aferritic stainless steel that forms the fuel side of the interconnect.Alternatively, the fuel side of the interconnect can be formed from themetallic material that forms an oxide having a dissociation pressuregreater than a partial pressure of oxygen proximate the fuel side of theinterconnect during operation of the SOFCs.

Another non-limiting embodiment provides an interconnect for SOFCs, theinterconnect comprising an oxidant side formed from a ferritic stainlesssteel and comprising at least one via comprising a scale comprising amanganese-chromate spinel on at least a portion of a surface thereof,and at least one gas flow channel comprising an aluminum-rich oxidescale on at least a portion of a surface thereof; and a fuel sideopposite the oxidant side, the fuel side comprising a metallic materialthat forms an oxide having a dissociation pressure greater than apartial pressure of oxygen proximate the fuel side of the interconnectduring operation of the SOFCs.

Still another non-limiting embodiment provides an interconnect for SOFCscomprising a gas-impermeable body, the gas-impermeable body being formedfrom a ferritic stainless steel and including a fuel side comprising avia and a gas flow channel, and an oxidant side opposite the fuel side,the oxidant side comprising a via and a gas flow channel, wherein ametallic material that forms an oxide having a dissociation pressuregreater than a partial pressure of oxygen proximate the fuel side of theinterconnect during operation of the SOFCs is connected to at least aportion of the ferritic stainless steel on fuel side of the body.

Referring again to FIG. 2, as previously discussed, in a typical PSOFC,an oxidant is introduced proximate a cathode 222 of one SOFC, and a fuelis introduced proximate an anode 224 of an adjacent another SOFC in thePSOFC. While in current generation SOFCs the pO₂ proximate the oxidantside of the interconnect is typically greater than 10⁻³ atmospheres, thepO₂ proximate the fuel side of an interconnect is substantially lowerand may vary depending upon factors such as the fuel type, fuelutilization level, and operating temperature of the PSOFC.

Referring now to FIG. 4, there is shown a graph illustrating thevariation in pO₂ levels on the fuel side of a typical current generationSOFC operating at temperatures ranging from 500° C. to 1000° C., atthree different fuel utilization levels (5%, 50% and 95%) using purehydrogen fuel (it should be appreciated, however, variations in pO₂levels on the fuel side of a typical current generation SOFC similar tothat shown in FIG. 4 will occur when other fuel types, for examplenatural gas, are employed). As illustrated in FIG. 4, the pO₂ on thefuel side of an interconnect can vary from about 10⁻³⁰ atmospheres togreater than 10⁻¹⁵ atmospheres, depending in part on the fuelutilization level and operating temperature employed. As previouslydiscussed, at the operating temperatures commonly employed in currentgeneration SOFCs, both stainless steel and most commonly used metalconductors will oxidize when exposed to pO₂ levels such as thoseproximate the oxidant side of an interconnect. Further, as previouslydiscussed, stainless steels will also generally oxidize when exposed topO₂ levels such as those proximate the fuel side of an interconnectduring operation of a current generation SOFC. However, since ascompared to most commonly used metal conductors, stainless steel formsrelatively slow growing and electrically conductive chromium oxide scaleon its surface, stainless steels have attracted interest for use informing interconnects for SOFCs. It will be appreciated by those skilledin the art that the variation in pO₂ on the fuel side a particular SOFCwill depend upon several factors, including without limitation, thedesign of the SOFC. Accordingly, FIG. 4 is intended for illustrationpurposes only and is not intended to limit the scope of the invention.

Referring now to FIG. 5, there is shown an Ellingham Diagram on whichthe conditions proximate the fuel side of a typical, current generationSOFC operating at fuel utilization levels of 5% and 95% and temperaturesof 500° C. and 1000° C. (which correlate to the pO₂ values as shownabove in FIG. 4) are highlighted. Metals that form an oxide having adissociation pressure that is greater than the pO₂ proximate the fuelside of an interconnect under the typical operating conditions(highlighted by box 50 in FIG. 5) are those metals that have anEllingham line (i.e., the equilibrium line for oxide formation) between500° C. and 1000° C. that lies completely outside and above box 50—forexample, metals such as copper, nickel, and cobalt. Iron, which has anEllingham line that intersects box 50, forms an oxide that has adissociation pressure that is greater than the pO₂ proximate the fuelside of the interconnect under some conditions, but less than the pO₂proximate the fuel side under other conditions. Chromium, which has anEllingham line that lies completely outside and below box 50, forms anoxide having a dissociation pressure that is less than the pO₂ proximatethe fuel side of the model interconnect under all the typicallyoperating conditions indicated in FIG. 5. Accordingly, it is expectedthat chromium will oxidize when exposed to the conditions proximate thefuel side of an interconnect during operation of a typical currentgeneration SOFC. It will be appreciated by those skilled in the art thatthe actual operating envelope for a given SOFC will depend upon severalfactors, including without limitation, the operating temperature, fuelutilization level, and fuel type employed. Accordingly, FIG. 5 isintended for illustration purposes only and is not intended to limit thescope of the invention.

While Fe—Cr ferritic stainless steels tend to oxidize when exposed tothe pO₂ levels near both the anode of a SOFC and the cathode of a SOFCduring operation of the PSOFC (as indicated above in FIG. 5) some metalswill not oxidize when exposed to the environment proximate the anode ofthe SOFC. Further, since the electrical resistivity of metals, such asbut not limited to copper, nickel and copper-nickel alloys, is generallylower than the electrical resistivity of the chromium oxide scale thatforms on a ferritic stainless steel interconnect during operation of theSOFCs, by employing a combination of a ferritic stainless steel (whichis optionally electropolished as discussed herein) and a metallicmaterial that forms an oxide having a dissociation pressure greater thanthe pO₂ proximate the fuel side of the interconnect during operation ofthe SOFCs, the oxidation properties of the interconnects according tovarious non-limiting embodiments disclosed herein may be tailored to theenvironmental conditions experienced by the interconnect duringoperation of the SOFCs.

For example, as shown in FIG. 3 a, although not limiting herein, theinterconnect (generally indicated as 310 a) may comprises a body 340 athat is formed from a ferritic stainless steel 344 a and that isgas-impermeable, so as to separate the fuel and oxidant gases flowing onfuel side 314 a and the oxidant side 316 a of interconnect 310 a,respectively. Further, according to this non-limiting example, ametallic material 342 a that forms an oxide having a dissociationpressure greater than a partial pressure of oxygen proximate the fuelside 314 a of the interconnect 310 a during operation of the solid oxidefuel cells may be connected to at least a portion of the ferriticstainless steel 344 a on fuel side 314 a of the body 340 a to reduce orprevent contact between the fuel gas and the ferritic stainless steel.For example, according to one non-limiting embodiment, metallic material342 a may be present as a gas-impermeable or solid layer that is bondedto the ferritic stainless steel 344 a on the fuel side 314 a of the body340 a. Methods of forming such a layer include, but are not limited to,plating and cladding.

Alternatively, according to another non-limiting embodiment, the fuelside of the interconnect can be formed from a metallic material thatforms an oxide having a dissociation pressure that is greater than apartial pressure of oxygen proximate the fuel side of the interconnectduring operation of the SOFCs. For example, as shown in FIG. 3 b, onenon-limiting embodiment provides an interconnect (generally indicated as310 b) for SOFCs comprising a composite body 340 b, the composite bodycomprising an oxidant side 316 b formed from a ferritic stainless steel344 b and a fuel side 314 b opposite the oxidant side, the fuel sidebeing formed from a metallic material 342 b that forms an oxide having adissociation pressure greater than a partial pressure of oxygenproximate the fuel side of the interconnect during operation of thesolid oxide fuel cells. According to this non-limiting example, body 340b may be gas-impermeable as previously discussed to provide forseparation of the gaseous reactants.

According to various non-limiting embodiments disclosed herein, themetallic material that forms an oxide having a dissociation pressurethat is greater than the partial pressure of oxygen proximate the fuelside of the interconnect may be selected from at least one of nickel andnickel alloys, copper and copper alloys, iron and iron alloys, cobaltand cobalt alloys, gold and gold alloys, and platinum and platinumalloys. According to certain non-limiting embodiments, the metallicmaterial may be nickel or a nickel alloy, copper or a copper alloy, or anickel-copper alloy. As used herein the term “nickel alloy” means analloy that contains nickel as its major component on a per elementbasis. As used herein the term “copper alloy” means an alloy thatcontains copper as its major component on a per element basis. Further,as used herein the term “nickel-copper alloy” means an alloy thatcontains essentially equal amounts of nickel and copper, and nickel andcopper are the major components of the alloy on a per element basis. Forexample, according to one non-limiting embodiment the metallic materialmay be a nickel alloy comprising up to 49 weight percent copper, up to49 weight percent iron, and nickel. One non-limiting example a suitablenickel alloy is AL 400™ alloy (generally designated UNS-N04400), whichis commercially available from Allegheny Ludlum Corporation ofPittsburgh, Pa. USA, and has a typical composition of 0.10 weightpercent carbon, 0.50 weight percent manganese, 0.005 weight percentphosphorus, 0.005 weight percent sulfur, 0.25 weight percent silicon,0.02 weight percent aluminum, 32 weight percent copper, 1 weight percentiron, the balance being nickel and impurities.

According to other non-limiting embodiments, the metallic material thatforms an oxide having a dissociation pressure that is greater than thepartial pressure of oxygen proximate the fuel side of the interconnectmay be a metallic material that forms an oxide having a dissociationpressure greater than at least 10⁻³⁰ atmospheres at a temperatureranging from 500° C. to 1000° C. or can be metallic material that formsan oxide having a dissociation pressure greater than 10⁻²⁵ atmospheresat a temperature ranging from 500° C. to 1000° C.

Optionally, although not shown in FIGS. 3 a and 3 b, the side of theinterconnect opposite the metallic material (i.e., the oxidant side ofthe interconnect) can comprise at least one via that when subjected toan oxidizing atmosphere at a temperature of at least 650° C. develops ascale comprising a manganese-chromate spinel on at least a portion of asurface thereof, and at least one gas flow channel that when subjectedto an oxidizing atmosphere at a temperature of at least 650° C. developsan aluminum-rich oxide scale on at least a portion of a surface thereof,wherein the aluminum-rich oxide scale comprises iron and chromium andhas a hematite structure. For example, according to this non-limitingembodiment, at least a portion of the oxidant side of the interconnectcan be selectively electropolished, or the entire oxidant side or theentire interconnect can be electropolished, and thereafter selectedsurfaces of the interconnect (for example the via surfaces) can beabraded to remove material from those surfaces.

Alternatively, a layer, such as a layer of a nickel-base superalloy orother material that has reasonable electrical properties and oxidationresistance in air, may be connected (for example, by cladding) to theside of the interconnect opposite the metallic material (i.e., theoxidant side of the interconnect). For example, according to onenon-limiting embodiment and as shown in FIG. 3 c, the interconnect(generally indicated as 310 c) may comprise a body 340 c that is formedfrom a ferritic stainless steel 344 c and has a fuel side 314 c and anoxidant side 316 c opposite the fuel side. A metallic material 342 cthat forms an oxide having a dissociation pressure greater than apartial pressure of oxygen proximate the fuel side 314 c of theinterconnect 310 c during operation of the SOFCs may be connected to atleast a portion of the ferritic stainless steel 344 c on the fuel side314 c of the body 340 c as previously discussed. Alternatively, althoughnot shown in FIG. 3 c, the body of the interconnect may be a compositebody as described above with respect to FIG. 3 b. Further, according tothis non-limiting embodiment, a layer 346 c of a nickel-base superalloyor other material that has similar electrical properties and oxidationresistance to nickel-base superalloys in air, may be connected to theoxidant side 316 c of the interconnect 310 c to reduce or preventoxidation of the ferritic stainless steel 344 c on the oxidant side 316c due to exposure to the environment proximate the cathode duringoperation of the SOFC. Examples nickel-base superalloys that may be usedin connection with this non-limiting embodiment include, withoutlimitation, Alloy 600 alloy (also designated UNS-N06600), Alloy 625(also designated UNS-N06625), and Alloy HX (also designated UNS-N06002).Other nickel-base superalloys having aluminum and silicon levels nogreater than 0.5 weight percent are also contemplated for use inconnection with this non-limiting embodiment, for example, Alloy 230(also designated UNS-N06230), which may include, in weight percent, upto 0.3 aluminum and up to 0.4 silicon.

While nickel-base superalloys generally have a good combination ofproperties for use in interconnects, such as good electricalconductivity and oxidation resistance, because nickel-base superalloyscan be expensive and generally have a CTE that is substantially higherthan the CTE ceramic electrodes, the use of nickel-base superalloys ininterconnect applications can be limited in certain applications.However, by balancing the thicknesses and CTEs of the various componentsof the interconnects, both the cost of the nickel-base superalloy andeffect of the relatively high CTE of the nickel-base superalloy can bemitigated. For example, according to one non-limiting embodiment, theferritic stainless steel component of the interconnect can comprise from60 to 80 percent of the total thickness of the interconnect, with themetallic material on the fuel side of the interconnect and nickel-basesuperalloy layer on the oxidant side of the interconnect each comprisingfrom 10 to 20 percent of the total thickness of the interconnect.Further, although not required, if metallic material on the fuel sidehas a CTE that is higher than the CTE of the nickel-base superalloylayer on the oxidant side, the metallic material can have a thicknessthat is less than the thickness of nickel-base superalloy layer, so asto evenly distribute the CTE mismatch on either side of theinterconnect. Conversely, if the nickel-base layer on the oxidant sideof the interconnect has a higher CTE than metallic material on the fuelside of the interconnect, the nickel-base superalloy layer can have athickness that is less than the thickness of metallic material on thefuel side. For example, although not limiting herein, if the metallicmaterial on the fuel side has a CTE that is 10% higher than the CTE ofthe nickel-base superalloy layer on the oxidant side, the metallicmaterial on the fuel side can have a thickness than is 10% less thanthat of the nickel-base superalloy.

With continued reference to FIG. 3 c, since metallic material 342 c andlayer 346 c prevent ferritic stainless steel 344 c from being directlyexposed to the environment surrounding the anode and the cathode,respectively, the oxidation resistance of the ferritic stainless steelaccording to these non-limiting embodiments has a relatively smallaffect on the overall electrical performance of the interconnect.Consequently, according to this non-limiting embodiment, ferriticstainless steels having low alloy and/or low chromium content, which aregenerally less expensive but have lower oxidation resistance then morehighly alloyed ferritic stainless steels, can be used to form body 340c. Non-limiting examples of low alloy and/or low chromium containingferritic stainless steels that can be used in conjunction with thesenon-limiting embodiments include ferritic stainless steels designatedType 409 (e.g., Fe-11 Cr ferritic stainless steels, also designated asUNS-S40900, UNS-S40920, and UNS-S40930) and Type 430 (e.g., Fe-16Crferritic stainless steels, also designated as UNS-S43000).

As previously discussed, although ferritic stainless steels tend tooxidize when exposed to the oxidant-rich environment near the cathode ofthe SOFC during operation (i.e., the environment proximate the oxidantside of the interconnect), the oxide scale formed on the surface of thestainless steel tends to be slow growing and electrically conductive.Thus, according to various non-limiting embodiments disclosed herein, atleast the portion of the interconnect that is in contact with theoxidant-rich environment near the cathode of the SOFC (i.e., the oxidantside of the interconnect) is formed from stainless steel, and inparticular, a ferritic stainless steel. Alternatively, an additionallayer, such as a layer of a nickel-base superalloy or other materialthat has reasonable oxidation resistance in air, may be connected (forexample, by cladding) to the oxidant-side of the interconnect asdiscussed above with respect to FIG. 3 c.

Referring now to FIG. 6, as previously discussed, ferritic stainlesssteels generally have lower CTEs than austenitic stainless steels, aswell as other materials that have been proposed for use as interconnectsfor SOFCs, such as the nickel-based alloys shown in FIG. 6. Sinceinterconnects for SOFCs generally must have a CTE that is sufficientlysimilar to the CTE of the ceramic electrodes within the SOFCs to ensurethe requisite structural integrity and gas-tightness of the assembly, itis generally desirable for the interconnects to be formed from amaterial that has a CTE that matches the CTEs of the electrodes asclosely as possible. Accordingly, by forming the interconnect bodiesfrom ferritic stainless steels, rather than austenitic stainless steelsor nickel-base superalloys which have higher CTEs than ferriticstainless steel, and by balancing the proportion of ferritic stainlesssteel to the metallic material which forms an oxide having adissociation pressure that is greater than the pO₂ proximate the fuelside of the interconnect during operation of an SOFC (and to thenickel-base superalloy layer, if present), the CTE of the interconnectsaccording to various non-limiting embodiments disclosed herein can beadjusted for compatibility with the ceramic electrodes of the SOFCs.

The CTE of the interconnects according to various non-limitingembodiments disclosed herein that comprise both a ferritic stainlesssteel and a metallic material that forms an oxide having a dissociationpressure that is greater than the pO₂ proximate the fuel side of theinterconnect during operation of an SOFC, and optionally comprise anadditional layer of a nickel-base superalloy on the oxidant side asdiscussed above with respect to FIG. 3 c, can be approximated accordingto the following equations:CTE _((I)) =CTE _((fss)) /X _((fss)) +CTE _((mm)) /X _((mm)) +CTE_((Ni)) /X _((Ni))  Eq. 1X _((fss)) =t _((fss))/(t _((fss)) +t _((mm)) +t _((Ni)))  Eq. 2X _((mm)) =t _((mm))/(t _((fss)) +t _((mm)) +t _((Ni)))  Eq. 3X _((Ni)) =t _((Ni))/(t _((fss)) +t _((mm)) +t _((Ni)))  Eq. 4wherein CTE_((I)) is the overall coefficient of thermal expansion of theinterconnect, CTE_((fss)) is the coefficient of thermal expansion of theferritic stainless steel, CTE_((mm)) is the coefficient of thermalexpansion of the metallic material which forms an oxide having adissociation pressure that is greater than the pO₂ proximate the fuelside of the interconnect during operation of an SOFC, and CTE_((Ni)) isthe coefficient of thermal expansion of the nickel-base superalloy. Thevariables X_((fss)), X_((mm)), and X_((Ni)) in Eq. 1 are given by Eqs.2-4 above, wherein t_((fss)) is the thickness of the ferritic stainlesssteel, t_((mm)) is the thickness of the metallic material, and t_((Ni))is the thickness of the layer of the nickel-base superalloy. In otherwords, CTE_((I)) is equal to a sum of the CTEs of each component (i.e.,the ferritic stainless steel, the metallic material, and the layer ofthe nickel-base superalloy if present) weighted by each component'sfraction of the total thickness of the interconnect. It will beappreciated that if the interconnect does not include a nickel-basesuperalloy layer on the oxidant side of the interconnect, the foregoingequations can be simplified to eliminate the terms related to thiscomponent.

For example, according to various non-limiting embodiments disclosedherein, the composition of the ferritic stainless steel, the compositionof the metallic material, the composition of the nickel-base superalloylayer on the oxidant side of the interconnect if present, and thefractional thicknesses of each of these components to the total combinedthickness can be selected such that the interconnect has an averagecoefficient of thermal expansion no greater than 17 ppm/K. According toother non-limiting embodiments, the ratios and materials can be chosenso as to provide an interconnect having a CTE no greater than 15 ppm/Kor no greater than 13 ppm/K. However, as discussed above, it isgenerally desirable to match the CTE of the interconnect to the CTE ofthe ceramic electrodes as closely as possible. Accordingly, theinterconnects according to various non-limiting embodiments disclosedherein can have any CTE necessary to provide for suitable performanceand reliability of the SOFCs or the PSOFC into which they areincorporated.

As discussed above, according to various non-limiting embodimentsdisclosed herein, the side of the interconnect opposite the metallicmaterial (i.e., the oxidant side of the interconnect) may beelectropolished. For example, although not limiting herein, theinterconnect may be electropolished by placing the interconnect in abath containing an electropolishing solution (which typically containsan acid and a carrier), electrically connecting the interconnect to acathode, and passing current between the interconnect and the cathode sothat material is electrochemically removed from the surface of theinterconnect. Further, although not limiting herein, the oxidant side ofthe interconnect may be selectively electropolished, for example, bymasking-off certain surfaces or regions of the interconnect (forexample, the via surfaces) prior to placing the interconnect into thebath containing the electropolishing solution so as to avoidelectropolishing of those surfaces. Alternatively, as previouslydiscussed, the entire oxidant side of the interconnect can beelectropolished and thereafter certain surfaces of the interconnect (forexample, the via surfaces) can be physically or chemically abraded toreduce or eliminate the effects of electropolishing at those surfaces.

Examples of ferritic stainless steels that can be used to form theinterconnects of various non-limiting embodiments disclosed hereininclude, but are not limited to, ferritic stainless steels comprising atleast 12 weight percent chromium. According to one non-limitingembodiment, the ferritic stainless steel may comprise at least 18 weightpercent chromium, may further comprise from 18 to 35 weight percentchromium, and may still further comprise from 20 to 28 weight percentchromium. Ferritic stainless steels comprising at least 20 weightpercent chromium are believed to be particularly useful in conjunctionwith various non-limiting embodiments disclosed herein, since such“high-chromium containing” ferritic stainless steels tend to form oxidescales having relatively low electrical resistivity. Specific,non-limiting examples of commercially available ferritic stainlesssteels comprising at least 20 weight percent chromium include AL 453™ferritic stainless steel and E-Brite® ferritic stainless steel.

According to still other non-limiting embodiments, the ferriticstainless steel can be a high-chromium containing ferritic stainlesssteel alloy that further comprises rare earth metal (“REM”) alloyingadditions. Although not limiting herein, it is contemplated that theaddition of REMs to a high-chromium containing alloy (such as the AL453™ ferritic stainless steel alloy discussed above) may result in theformation of an adherent, slow-growing chromium-oxide scale at hightemperatures. REM in the form of mischmetal may be added duringproduction of the ferritic stainless steel. Mischmetal is an availablecommercial form of mixed REMs and can be obtained with differentcompositions having known concentrations of the REMs cerium, lanthanumand praseodymium. For example, a common mischmetal form used insteelmaking is nominally 50Ce-30La-20Pr by weight.

According to other non-limiting embodiments, the ferritic stainlesssteel can be low in alloying elements that form continuous oxides havingelectrical resistivity that is greater than the electrical resistivityof chromium oxide. Non-limiting examples of alloying elements that formcontinuous oxides having electrical resistivity that is greater than theelectrical resistivity of chromium oxide include aluminum and silicon.For example, stainless steels comprising less than 0.1 weight percentaluminum and/or less than 0.1 weight percent silicon; and ferriticstainless steels comprising from 0 to 0.005 weight percent aluminumand/or from 0 to 0.005 weight percent silicon can be used in accordancewith various non-limiting embodiments disclosed herein. Although notlimiting herein, it is contemplated that by reducing the amount ofalloying elements in the ferritic stainless steel that form continuousoxides having an electrical resistivity greater than that of chromiumoxide, the chromium-rich oxide and/or manganese-chromate spinel scalethat forms on a ferritic stainless steel interconnect during operationof a PSOFC may have improved electrical properties as compare tointerconnects made using conventional ferritic stainless steels.

Further, the ferritic stainless steels according to various non-limitingembodiments disclosed herein can comprise from greater than 1 weightpercent to 2 weight percent manganese. As previously discussed,manganese can segregate to the surface of a ferritic stainless steelduring oxidation at high temperatures thereby forming a scale comprisinga manganese-chromate spinel (e.g., MnCr₂O₄). As discussed above, theformation of a scale comprising manganese-chromate spinel on the surfaceof the ferritic stainless steel during oxidation may reduce chromiummigration from the surface of the ferritic stainless steel. While notlimiting herein, it is contemplated that the maximum resistance tochromium migration may be achieved when the manganese-chromate spinelformed on the ferritic stainless steel surface is saturated or nearlysaturated with manganese, which generally requires a manganese contentof greater than 1 weight percent of the ferritic stainless steel.Therefore, the ferritic stainless steels according to certainnon-limiting embodiments disclosed herein may comprise greater than 1weight percent manganese, may further comprise at least 1.5 weightpercent manganese, and may still further comprises at least 1.6 weightpercent manganese.

However, since the overall thickness of the scale on the surface of theferritic stainless steel tends to increase with increasing manganesecontent, for certain applications it may be desirable to prevent theformation of thick scales on the surface of the ferritic stainless steel(at least in the electrical contact areas) in order to keep the ASR ofthe interconnect as low as practicable. Therefore, according to variousnon-limiting embodiments disclosed herein, the amount of manganesepresent in the ferritic stainless steel may range from greater than 1weight percent to 2 weight percent, may further range from at least 1.5weight percent to 2 weight percent, and may still further range from 1.6weight percent to 2 weight percent.

Non-limiting examples of ferritic stainless steels that may be used toform interconnects according to various non-limiting embodimentsdisclosed herein and that have a high-chromium content, that optionallycontain REMs, and that are both low in alloying elements that formcontinuous oxides having electrical resistivity that is greater than theelectrical resistivity of chromium oxide and contain from greater than 1to 2 weight percent manganese are set forth below in Table II. TABLE IIComposition Composition Composition Composition 1 (Weight 2 (Weight 3(Weight 4 (Weight Element Percent) Percent) Percent) Percent) Aluminum 0to <0.1 0 to 0.05 0 to 0.05 0 to 0.05 Silicon 0 to <0.1 0 to 0.05 0 to0.05 0 to 0.05 Chromium 21 to 35 21 to 24 23 to 27 23 to 27 Manganese >1to 2 >1 to 2 >1 to 2 >1 to 2 Carbon 0.002 to 0.1 0.002 to 0.1 0.002 to0.1 0.002 to 0.1 Nitrogen 0 to 0.04 0 to 0.04 0 to 0.04 0 to 0.04Molybdenum 0 to 1 0 to 1 0 to 1 0.75 to 1 Nickel 0 to 0.5 0 to 0.3 0 to0.3 0 to 0.3 Lanthanum 0 to 0.05 0.02 to 0.04 0 to 0.05 0 to 0.05 Cerium0 to 0.1 * 0 to 0.1 0 to 0.1 Zirconium 0 to 0.1 0 to 0.1 0 to 0.1 0 to0.05 Titanium 0 to 0.5 0 to 0.1 0 to 0.5 ** Tantalum 0 to 0.1 0 to 0.1 0to 0.1 ** Niobium 0 to 0.2 0 to 0.1 0.05 to 0.2 ** Iron & BalanceBalance Balance Balance Impurities* Weight Percent Cerium + Weight Percent Lanthanum ranges from 0.03 to0.06.** Given by: 0.4 weight percent ≦ [% Nb + % Ti + ½(% Ta)] ≦ 1 weightpercent.

With respect to Composition 1 (above), the amount of aluminum and/orsilicon in the ferritic stainless steel compositions set forth in TableII above may range from 0 to 0.05 weight percent. Further, with respectto Compositions 1-4 (above), the amount of aluminum and/or silicon mayrange from 0.005 to 0.05 weight percent. Still further, with respect toCompositions 1-4, the amount of manganese may range from 1.5 to 2 weightpercent, and may further range from 1.6 to 2 weight percent.

As previously discussed, various non-limiting embodiments of the presentdisclosure relate to ferritic stainless steels, and in particular,ferritic stainless steels that may be useful in fabricatinginterconnects according to various non-limiting embodiments disclosedherein. For example, one non-limiting embodiment provides a ferriticstainless steel comprising Composition 1 as set forth above in Table II.Further, according to this non-limiting embodiment, the ferriticstainless steel can comprise from 0 to 0.05 weight percent aluminumand/or silicon or from 0.005 to 0.05 weight percent aluminum and/orsilicon. Still further, the ferritic stainless steel according to thisnon-limiting embodiment may comprise from 1.5 to 2 weight percentmanganese or from 1.6 to 2 weight percent manganese.

Another non-limiting embodiment provides a ferritic stainless steelcomprising Composition 2 as set forth above in Table II. Further,according to this non-limiting embodiment, the ferritic stainless steelcan comprise from 0.005 to 0.05 weight percent aluminum and/or silicon.Still further, the ferritic stainless steel according to thisnon-limiting embodiment may comprise from 1.5 to 2 weight percentmanganese or from 1.6 to 2 weight percent manganese.

Another non-limiting embodiment provides a ferritic stainless steelcomprising Composition 3 as set forth above in Table II. Further,according to this non-limiting embodiment, the ferritic stainless steelcan comprise from 0.005 to 0.05 weight percent aluminum and/or silicon.Still further, the ferritic stainless steel according to thisnon-limiting embodiment may comprise from 1.5 to 2 weight percentmanganese or from 1.6 to 2 weight percent manganese.

Another non-limiting embodiment provides a ferritic stainless steelcomprising Composition 4 as set forth above in Table II. Further,according to this non-limiting embodiment, the ferritic stainless steelcan comprise from 0.005 to 0.05 weight percent aluminum and/or silicon.Still further, the ferritic stainless steel according to thisnon-limiting embodiment may comprise from 1.5 to 2 weight percentmanganese or from 1.6 to 2 weight percent manganese. Additionally, theamounts of one or more of niobium, titanium, and/or tantalum may befurther chosen to satisfy the equation 0.5 weight percent≦[% Nb+% Ti+½(%Ta)]≦1 weight percent, or may be chosen to satisfy the equation 0.5weight percent≦[% Nb+% Ti+½(% Ta)]≦0.75 weight percent. Further,according to various non-limiting embodiments, the amount of titanium inComposition 4 can be no greater than 0.5 weight percent.

The ferritic stainless steels according to various non-limitingembodiments disclosed herein can also be essentially free of silver,calcium oxide, and/or titanium. As used herein with respect to thecomposition of the ferritic stainless steel, the term “essentially freeof” means that no more than impurity or residual levels of the specifiedelement(s) are present in the ferritic stainless steel.

Other non-limiting embodiments disclosed herein provide interconnectsmade from the ferritic stainless steels set forth above. For example,one non-limiting embodiment provides an interconnect for SOFCs, theinterconnect comprising a ferritic stainless steel comprisingComposition 1. Further, as discussed above, according to thisnon-limiting embodiment, the ferritic stainless steel can comprise from0 to 0.05 weight percent aluminum and/or silicon or from 0.005 to 0.05weight percent aluminum and/or silicon. Still further, the ferriticstainless steel according to this non-limiting embodiment may comprisefrom 1.5 to 2 weight percent manganese or from 1.6 to 2 weight percentmanganese.

Another non-limiting embodiment provides an interconnect for SOFCs, theinterconnect comprising a ferritic stainless steel comprising acomposition as set forth for Composition 2, 3 or 4 above. Further, asdiscussed above, according to this non-limiting embodiment, the ferriticstainless steel can comprise from 0.005 to 0.05 weight percent aluminumand/or silicon. Still further, the ferritic stainless steel may comprisefrom 1.5 to 2 weight percent manganese or from 1.6 to 2 weight percentmanganese.

As previously discussed, various non-limiting embodiments disclosedherein relate to PSOFCs made using the interconnects according to any ofthe foregoing non-limiting embodiments. For example, one non-limitingembodiment provides a PSOFC comprising a first solid oxide fuel cellhaving an anode, a cathode, and a solid oxide electrolyte between theanode and the cathode; a second solid oxide fuel cell having an anode, acathode, and a solid oxide electrolyte between the anode and thecathode, the second solid oxide fuel cell being positioned such that theanode of the first solid oxide fuel cell is adjacent the cathode of thesecond solid oxide fuel cell, and an interconnect interposed between thefirst and second solid oxide fuel cells. According to this non-limitingembodiment, the interconnect is formed from a ferritic stainless steeland comprises a fuel side positioned adjacent the anode of the firstsolid oxide fuel cell, the fuel side comprising at least one via and atleast one gas flow channel, and an oxidant side opposite the fuel sideof the interconnect and adjacent the cathode of the second solid oxidefuel cell, the oxidant side of the interconnect being formed from aferritic stainless steel and comprising at least one via that whensubjected to an oxidizing atmosphere at a temperature of at least 650°C. develops a scale comprising a manganese-chromate spinel on at least aportion of a surface thereof and at least one gas flow channel that whensubjected to an oxidizing atmosphere at a temperature of at least 650°C. develops an aluminum-rich oxide scale on at least a portion of asurface thereof, the aluminum-rich oxide scale comprising iron andchromium and having a hematite structure.

Further, according to various non-limiting embodiments disclosed herein,the fuel side of the interconnect can comprise a ferritic stainlesssteel and at least one via of the fuel side may develop a scalecomprising a manganese-chromate spinel on at least a portion of asurface thereof when subjected to an oxidizing atmosphere at atemperature of at least 650° C. Still further, at least one gas flowchannel of the fuel side may develop an aluminum-rich oxide scale on atleast a portion of a surface thereof, the aluminum-rich oxide scalecomprising iron and chromium and having a hematite structure, whensubjected to an oxidizing atmosphere at a temperature of at least 650°C.

Alternatively, the fuel side of the interconnect may comprise a ferriticstainless steel having a metallic material that forms an oxide having adissociation pressure greater than the pO₂ proximate the fuel side ofthe interconnect during operation of the PSOFC connected to at least aportion of a surface thereof. Still further, the fuel side can be formedfrom a metallic material that forms an oxide having a dissociationpressure greater than the pO₂ proximate the fuel side of theinterconnect during operation of the PSOFC. Non-limiting examples ofsuitable ferritic stainless steels and metallic materials that formoxides having a dissociation pressure greater than the pO₂ proximate thefuel side of the interconnect during operation of the PSOFC are setforth above in detail.

Another non-limiting embodiment provides a PSOFC comprising a firstsolid oxide fuel cell having an anode, a cathode, and a solid oxideelectrolyte between the anode and the cathode; a second solid oxide fuelcell having an anode, a cathode and a solid oxide electrolyte betweenthe anode and the cathode, the second solid oxide fuel cell beingpositioned such that the anode of the first solid oxide fuel cell isadjacent the cathode of the second solid oxide fuel cell; and aninterconnect interposed between the first and second solid oxide fuelcells. According to this non-limiting embodiment, the interconnect hasan oxidant side comprising a ferritic stainless steel, and a fuel sideopposite the oxidant side of the interconnect, the fuel side comprisinga metallic material that forms an oxide having a dissociation pressuregreater than a pO₂ proximate the fuel side of the interconnect duringoperation of the planar solid oxide fuel cell. More particularly,according to this non-limiting embodiment, the interconnect isinterposed between the first solid oxide fuel cell and the second solidoxide fuel cell such that the fuel side of the interconnect is adjacentthe anode of the first solid oxide fuel cell, and the oxidant side ofthe interconnect is adjacent the cathode of the second solid oxide fuelcell. Further, according to this non-limiting embodiment, the fuel sideof the interconnect may be formed from a ferritic stainless steel andthe metallic material that forms an oxide having a dissociation pressuregreater than a pO₂ proximate the fuel side of the interconnect duringoperation of the PSOFC connected to at least a portion of the ferriticstainless steel.

Methods of making interconnects according to various non-limitingembodiments disclosed herein will now be discussed. One non-limitingembodiment provides a method of making an interconnect comprisingforming an interconnect from a ferritic stainless steel, theinterconnect having a fuel side and an oxidant side opposite the fuelside, each of the fuel side and oxidant side comprising a via and a gasflow channel, and electropolishing at least a portion of at least onegas flow channel of the oxidant side of the interconnect.

For example, according to this non-limiting embodiment, electropolishingcan comprise selectively electropolishing at least a portion of at leastone gas flow channel of the oxidant side of the interconnect. Moreparticularly, according to this non-limiting embodiment, selectiveelectropolishing may comprise masking those portions of the interconnectthat are not to be electropolished using a masking material. Forexample, the vias of the oxidant side, the entire fuel side or, if thegas flow channels of the fuel side are to be selectivelyelectropolished, the vias of the fuel side can be masked to avoidelectropolishing of those portions. Non-limiting examples of suitablemasking materials include photoresists, waxes, and masking tapes.Thereafter, the interconnect can be electropolished as discussed above.After electropolishing, the masking material can be removed by knownmethods.

Optionally, before or after selective electropolishing, a metallicmaterial that forms an oxide having a dissociation pressure greater thanthe pO₂ proximate the fuel side of the interconnect during operation ofthe PSOFC can be connected to the fuel side. Alternatively, the metallicmaterial can be connected to the ferritic stainless steel prior toforming the interconnect. Non-limiting methods connecting metallicmaterials that form oxides having a dissociation pressure greater thanthe pO₂ proximate the fuel side of the interconnect during operation ofthe PSOFC are described below in more detail.

Another non-limiting embodiment provides a method of making aninterconnect comprising forming an interconnect from a ferriticstainless steel, the interconnect having a fuel side and an oxidant sideopposite the fuel side, each of the fuel side and oxidant sidecomprising a via and a gas flow channel, electropolishing at least aportion of the oxidant side of the interconnect, and physically orchemically removing material from at least one electropolished surfaceof at least one via of the oxidant side of the interconnect. Optionally,a metallic material that forms an oxide having a dissociation pressuregreater than the pO₂ proximate the fuel side of the interconnect duringoperation of the PSOFC can be connected to the fuel side of theinterconnect after the interconnect is formed, either before or afterelectropolishing. Alternatively, the metallic material can be connectedto the ferritic stainless steel prior to forming the interconnect.

Another non-limiting embodiment provides a method of making aninterconnect comprising connecting a metallic material on at least aportion of a surface of the ferritic stainless steel sheet material, themetallic material being nickel or a nickel alloy, copper or a copperalloy, or a nickel-copper alloy, and forming an interconnect from theferritic stainless steel sheet material, the interconnect having aoxidant side comprising ferritic stainless steel and a fuel sideopposite the oxidant side comprising the metallic material, each of theoxidant side and fuel side of the interconnect comprising a via and agas flow channel. As discussed in more detail below, according to thisnon-limiting embodiment, forming the interconnect can occur prior to,during, or after connecting the metallic material to at least a portionof the ferritic stainless steel.

Optionally, according to the forgoing non-limiting embodiment, at leasta portion of at least one gas flow channel of the oxidant side of theinterconnect can be electropolished, for example by selectiveelectropolishing; or the entire oxidant side can be electropolished and,thereafter, material may be removed from a portion of at least oneelectropolished via surface (as previously discussed). Alternatively,the ferritic stainless steel can be electropolished before connectingthe metallic material thereto or after connecting the metallic materialbut prior to forming the interconnect.

Still further, according to various non-limiting embodiments, a layer ofa nickel-base superalloy can be connected to a surface of the ferriticstainless steel opposite the surface to which the metallic material isconnected prior to forming the interconnect, for example by hot rollingor cold roll bonding. For example, according to one non-limitingembodiment, a ferritic stainless steel slab can interposed between alayer of the metallic material and a layer of the nickel-basesuperalloy. Thereafter, the stack can be clad together and processed tothe desired thickness by hot rolling. Alternatively, each of thematerials can be processed into sheets or coils having a desiredthickness prior to being clad together in a cold bonding process.

According to various non-limiting embodiments disclosed herein, theferritic stainless steel can be a ferritic stainless steel sheetmaterial that has been rolled to a desired thickness or finished gauge.For example, the ferritic stainless steel sheet material can be formedfrom a slab (or another coil or sheet) having a thickness that isgreater than the desired thickness by rolling the slab, coil or sheet toa desired thickness using one or more conventional hot and/or coldrolling processes.

Methods of connecting a metallic material to at least a portion of aferritic stainless steel according to various non-limiting embodimentsdisclosed herein include, without limitation, plating, cladding,fastening, joining, and combinations thereof. As used herein the term“plating” means building up of one material on another. Non-limitingexamples of suitable plating techniques include, without limitation,electroplating, electroless plating, as well as other physical orchemical deposition techniques (such as but not limited to sputtering,plasma vapor deposition, chemical vapor deposition, and thermal or flamespraying) that involve the build up of one material on another.

As used herein the term “cladding” means bringing two or more materialsinto direct contact with each other using pressure or pressure andtemperature. Non-limiting examples of suitable cladding methods include,without limitation, hot and cold roll bonding, as well as otherthermo-mechanical bonding techniques such as explosive bonding andforging.

As used herein the term “fastening” means mechanically connecting orinterlocking two or more materials, such as with a fastener or someother attachment device or interlocking geometry. For example, althoughnot limiting herein, the metallic material can be fastened to theferritic stainless steel by crimping the metallic material and theferritic stainless steel together to interlock the two materials.

As used herein the term “joining” means bonding two materials togethersuch as by welding, brazing or soldering. For example, according to onenon-limiting embodiment, the metallic material can be brazed to theferritic stainless steel.

According to one specific non-limiting embodiment, connecting themetallic material to at least a portion of the ferritic stainless steelcomprises at least one of plating the metallic material on at least aportion of a surface of the stainless steel and cladding the metallicmaterial on at least a portion of a surface of the stainless steel.

Non-limiting methods of forming an interconnect that can be used inconjunction with various non-limiting embodiments disclosed hereininclude forging, stamping, coining, machining, chemically milling andcombinations thereof. According to one non-limiting embodiment, formingthe interconnect can comprise, for example, stamping a sheet of ferriticstainless steel into the desired geometry. Alternatively, according tothis non-limiting embodiment, the interconnect can be formed using acombination of stamping and machining. According to another non-limitingembodiment forming the interconnect may comprise providing a ferriticstainless steel blank, applying a masking material to those portions ofthe ferritic stainless steel blank that are to be retained, andchemically milling the blank to remove material from the unmaskedportion of the blank to form an interconnect having at least one via andat least one gas flow channel. Thereafter, the interconnect can beelectropolished and after electropolishing the masking material. Forexample, according to this non-limiting embodiment, the portions of theferritic stainless steel blank that are masked may be those portions ofthe blank that form the vias in the interconnect. These and othermethods of forming interconnects are known in the art.

As previously discussed, according to various non-limiting embodimentsdisclosed herein, forming the interconnect can occur prior to, during,or after connecting the metallic material to at least a portion of theferritic stainless steel. For example, according to one non-limitingembodiment, wherein forming the interconnect occurs prior to connectingthe metallic material to the ferritic stainless steel, the method ofmaking an interconnect can comprise providing a ferritic stainless steelsheet material, forming an interconnect from the ferritic stainlesssteel sheet material, and plating a metallic material on at least aportion of at least one surface of the interconnect, the metallicmaterial being nickel or a nickel alloy, copper or a copper alloy, or anickel-copper alloy. Optionally, according to this non-limitingembodiment, a layer of a nickel-base superalloy may be clad to theferritic stainless steel prior to forming the interconnect such thatafter forming, the interconnect has a first side comprising thenickel-base superalloy and a second side opposite the first sidecomprising the ferritic stainless steel. Thereafter, the metallicmaterial may be plated on the ferritic stainless steel of the secondside of the interconnect.

According to another non-limiting embodiment, wherein forming theinterconnect occurs after the metallic material is connected to theferritic stainless steel, the method of making an interconnect cancomprise providing a ferritic stainless steel sheet material, cladding ametallic material on at least a portion of at least one surface of theferritic stainless steel sheet material, the metallic material beingnickel or a nickel alloy, copper or a copper alloy, or a nickel-copperalloy; and forming an interconnect from the clad ferritic stainlesssteel sheet material. Further, according to this non-limitingembodiment, prior to forming the interconnect from the clad ferriticstainless steel sheet material, the clad ferritic stainless steel sheetmaterial can be processed to a finished gauge, for example, by rolling.Optionally, according to this non-limiting embodiment, the metallicmaterial can be clad onto a first surface of the ferritic stainlesssteel and a layer of a nickel-base superalloy can be clad to a secondsurface of the ferritic stainless steel that is opposite the firstsurface.

According to still another non-limiting embodiment, wherein forming theinterconnect occurs at essentially the same time as connecting themetallic material to the ferritic stainless steel, the method of makingan interconnect can comprise providing a ferritic stainless steel sheetmaterial in a die, placing a sheet or foil of a metallic material nextto at least a portion of the ferritic stainless steel sheet material,the metallic material being nickel or a nickel alloy, copper or a copperalloy, or a nickel-copper alloy; and forming the interconnect by forgingunder heat and pressure such that the ferritic stainless steel and themetallic material are simultaneously formed into an interconnect andclad together. Optionally, according to this non-limiting embodiment,the sheet or foil of the metallic material can be placed next to a firstside of the ferritic stainless steel sheet material, and a sheet or foilof a nickel-base superalloy can be placed next to a second side of theferritic stainless steel, opposite the first side, and thereafter thestack can be forged to form the interconnect.

The following non-limiting examples are intended to facilitateunderstanding of various aspects of the present disclosure and are notintended to be limiting herein.

EXAMPLES Example 1

Three samples of each of the two ferritic stainless steels were obtainedand tested as follows. The first set of three ferritic stainless steelsamples were samples of AL 453™ alloy (the nominal composition for whichalloy is set forth above in Table I). These three samples wereelectropolished prior to testing as discussed below. The second set ofthree ferritic stainless steel samples were samples of E-BRITE® alloy(the nominal composition for which alloy is set forth above in Table I)and were tested without electropolishing.

Electropolishing of the AL 453™ alloy samples was conducted as follows.The samples to be electropolished were ground with 240 grit siliconcarbide wet/dry metallographic grinding paper. The samples were thenimmersed in an aqueous acid solution having the following composition:25% sulfuric acid, 47% phosphoric acid, and 28% glycolic acid. Thesolution was held at approximately 175° F. and electropolishing wascarried out by passing a current of 1 Amp./in² through the sample for 20minutes, with the samples being flipped every 5 minutes.

The oxidation resistance of each sample was then evaluated by thermaloxidation cycling. During each thermal oxidation cycle, the samples wereexposed to air containing 7% water vapor at a temperature of 760° C.,nominally for 160 hours. The cyclic exposure was repeated untilachieving a total exposure time of 4000 hours. The weight of each samplewas measured after each thermal oxidation cycle and recorded. Theresults of this testing are presented in FIG. 7 as a plot of weightchange per sample surface area (mg/cm²) vs. exposure time (hours) foreach sample tested.

As can be seen from FIG. 7, each of the electropolished samples of AL453™ alloy gained weight throughout the exposure period, whereas thenon-electropolished samples (i.e., the E-BRITE® alloy samples) lostweight after approximately 1000 hours of exposure, and continued to loseweight throughout the remaining exposure period. This weight loss isbelieved to be attributable to the evolution of chromium-bearing vaporspecies from the surface of the sample during testing.

Example 2

A sample of AL 453™ alloy (the nominal composition for which alloy isset forth above in Table I) was electropolished as described above inExample 1. Thereafter, a portion of the electropolished surface wasmechanically abraded using a diamond-tipped stylus. The sample was thenplaced in a furnace and heated at 875° C. for 72 hours under air tocause an oxide scale to form on the sample.

After exposure, the sample was observed using a scanning electronmicroscope and the characteristic x-rays of several elements were mappedto show the relative amounts of the various elements at or near thesurface of the sample. FIG. 8 a is the secondary electron image of thesample and FIGS. 8 b-8 e are characteristic x-ray maps of the same areashown in FIG. 8 a obtained using the characteristic x-rays of theelements indicated. In particular, FIG. 8 b is an x-ray mapping of thecharacteristic x-rays of chromium, FIG. 8 c is an x-ray mapping of thecharacteristic x-rays of iron, FIG. 8 d is an x-ray mapping of thecharacteristic x-rays of aluminum, and FIG. 8 e is an x-ray mapping ofthe characteristic x-rays of manganese.

As can be seen from this series of images, the portion of theelectropolished surface that was abraded using the diamond-tipped stylushad a higher concentration of chromium and manganese, indicating theformation of a scale of a manganese-chromate spinel in this portion. Incontrast, the portions of the electropolished surface that were notabraded had higher concentrations of iron and aluminum, indicating thatan aluminum-rich oxide scale developed in these portions and remainedintact throughout the oxidation testing. No substantial chromiummigration to the aluminum-rich oxide surface was observed.

It is to be understood that the present description illustrates aspectsof the invention relevant to a clear understanding of the invention.Certain aspects of the invention that would be apparent to those ofordinary skill in the art and that, therefore, would not facilitate abetter understanding of the invention have not been presented in orderto simplify the present description. Although the present invention hasbeen described in connection with certain embodiments, the presentinvention is not limited to the particular embodiments disclosed, but isintended to cover modifications that are within the spirit and scope ofthe invention, as defined by the appended claims.

1. A ferritic stainless steel comprising, in weight percent, from 0 toless than 0.1 aluminum, from 0 to less than 0.1 silicon, from 21 to 35chromium, greater than 1 to 2 manganese, from 0.002 to 0.1 carbon, from0 to 0.04 nitrogen, from 0 to 1 molybdenum, from 0 to 0.5 nickel, from 0to 0.05 lanthanum, from 0 to 0.1 cerium, from 0 to 0.1 zirconium, from 0to 0.5 titanium, from 0 to 0.1 tantalum, from 0 to 0.2 niobium, iron andimpurities.
 2. The ferritic stainless steel of claim 1 wherein theferritic stainless steel comprises: from 0 to 0.05 weight percentaluminum, and from 0 to 0.05 weight percent silicon.
 3. The ferriticstainless steel of claim 1 wherein the ferritic stainless steelcomprises: from 0.005 to 0.05 weight percent aluminum, and from 0.005 to0.05 weight percent silicon.
 4. The ferritic stainless steel of claim 1wherein the ferritic stainless steel comprises from 1.5 to 2.0 weightpercent manganese.
 5. The ferritic stainless steel of claim 1 whereinthe ferritic stainless steel comprises from 1.6 to 2.0 weight percentmanganese.
 6. The ferritic stainless steel of claim 1 wherein theferritic stainless steel comprises, in weight percent, from 0 to 0.05aluminum, from 0 to 0.05 silicon, from 21 to 24 chromium, greater than 1to 2 manganese, from 0.002 to 0.1 carbon, from 0 to 0.04 nitrogen, from0 to 1 molybdenum, from 0 to 0.3 nickel, from 0.02 to 0.04 lanthanum,from 0 to 0.1 zirconium, from 0 to 0.1 titanium, from 0 to 0.1 tantalum,from 0 to 0.1 niobium, cerium, iron and impurities, wherein a sum of theweight percent cerium and the weight percent lanthanum ranges from 0.03to 0.06.
 7. The ferritic stainless steel of claim 6 wherein the ferriticstainless steel comprises: from 0.005 to 0.05 weight percent aluminum,and from 0.005 to 0.05 weight percent silicon.
 8. The ferritic stainlesssteel of claim 6 wherein manganese ranges from 1.5 to 2 weight percent.9. The ferritic stainless steel of claim 6 wherein manganese ranges from1.6 to 2 weight percent.
 10. The ferritic stainless steel of claim 1wherein the ferritic stainless steel comprises, in weight percent, from0 to 0.05 aluminum, from 0 to 0.05 silicon, from 23 to 27 chromium,greater than 1 to 2 manganese, from 0.002 to 0.1 carbon, from 0 to 0.04nitrogen, from 0 to 1 molybdenum, from 0 to 0.3 nickel, from 0 to 0.05lanthanum, from 0 to 0.1 cerium, from 0 to 0.1 zirconium, from 0 to 0.5titanium, from 0 to 0.1 tantalum, from 0.05 to 0.2 niobium, iron andimpurities.
 11. The ferritic stainless steel of claim 10 wherein theferritic stainless steel comprises: from 0.005 to 0.05 weight percentaluminum, and from 0.005 to 0.05 weight percent silicon.
 12. Theferritic stainless steel of claim 10 wherein manganese ranges from 1.5to 2 weight percent.
 13. The ferritic stainless steel of claim 10wherein manganese ranges from 1.6 to 2 weight percent.
 14. The ferriticstainless steel of claim 1 wherein the ferritic stainless steel isessentially free of silver, calcium oxide, and titanium.
 15. Aninterconnect comprising a ferritic stainless steel according to claim 1.16. A ferritic stainless steel comprising, in weight percent, from 0 to0.05 aluminum, from 0 to 0.05 silicon, from 23 to 27 chromium, greaterthan 1 to 2 manganese, from 0.002 to 0.1 carbon, from 0 to 0.04nitrogen, from 0.75 to 1 molybdenum, from 0 to 0.3 nickel, from 0 to0.05 lanthanum, from 0 to 0.1 cerium, from 0 to 0.05 zirconium, anamount of at least one of titanium, tantalum and niobium, wherein theamounts of titanium, tantalum and niobium satisfy the equation:0.4 weight percent≦[% Nb+% Ti+½(% Ta)]≦1 weight percent, iron andimpurities.
 17. The ferritic stainless steel of claim 16 wherein theferritic stainless steel comprises: from 0.005 to 0.05 weight percentaluminum, and from 0.005 to 0.05 weight percent silicon.
 18. Theferritic stainless steel of claim 16 wherein manganese ranges from 1.5to 2 weight percent.
 19. The ferritic stainless steel of claim 16wherein manganese ranges from 1.6 to 2 weight percent.
 20. The ferriticstainless steel of claim 16 wherein the ferritic stainless steelcomprises no greater than 0.5 weight percent titanium.
 21. The ferriticstainless steel of claim 16 wherein the amounts of titanium, tantalumand niobium satisfy the equation:0.5 weight percent≦[% Nb+% Ti+½(% Ta)]≦1 weight percent.
 22. Theferritic stainless steel of claim 16 wherein the ferritic stainlesssteel is essentially free of silver, calcium oxide, and titanium.
 23. Aninterconnect comprising a ferritic stainless steel according to claim16.